1. Field Of The Invention
The present invention is directed to a method of producing an extruder barrel. More particularly the invention relates to a method of producing an extruder barrel by forming two complementary sections and welding the sections together by electron beam welding to form the extruder barrel.
2. Description Of The Background Art
Extruders are widely used machines in many diverse industries. For example, extruders are typically used in the plastic and rubber fields for forming molded plastic bodies. Extruders provide a convenient and efficient means for melting thermoplastic polymers, mixing components and generating sufficient pressure for injection molding or forming continuous length articles. Extruders are particularly effective in generating the high pressures needed to foam or expand the extruded material when extruded at atmospheric pressure.
Extruders typically include an elongated barrel defining a material inlet at one end and a treated material outlet at the opposite end. Within the bore of the extruder barrel is positioned one or more screws to mix and convey the material through the length of the barrel. The screws include a variety of mixing paddles and spiralling threads of selected pitches to convey the material and generate the pressure needed for extrusion. Under ideal conditions, the extruder screws tend to float within the material as the screws rotate.
Extruders may have a single rotating screw or a plurality of juxtaposed rotating screws. In order to achieve the desired pressures within the barrel, the screws must be closely spaced to the inner wall of the extruder barrel to prevent material from moving backwards in the barrel. Single screw extruders are generally considered to have limited uses in industry due to their limitations in producing high pressures and mixing of components. Twin screw extruders are the more commonly used extruders in the plastics industry due to the ability to generate high pressures to enable extrusion of viscous materials at high rates and pressures. Twin screw extruders have the further advantage of providing more uniform discharge and are less prone to extruder surging.
Twin screw extruders have a pair of parallel juxtaposed elongated screws which are carried within an extruder barrel having a complementary internal bore. The screws may be either corotating or counter-rotating, with the counter-rotating screws being more common. Counter-rotating screws provide better control of mixing and pressure within the extruder barrel. Single screw extruders tend to result in considerable forward and backward movement of the material along the length of the barrel. This often results in slippage of the material, particularly with highly viscous materials. Such slippage does not typically occur or is less likely with twin screw extruders.
In counter-rotating twin screw extruders, a high pressure zone occurs in the area between the screws. This high pressure zone tends to urge the screws outwardly away from each other. Although under ideal conditions the rotating screws float in the material, the screws flex slightly outward as the pressure in this high pressure zone increases. This flexing is more pronounced as the length of the screws increase since the screws are typically supported only at the end in bearings or bushings. The results of this flexing is to produce wear on the extruder screws and the inner wall of the extruder barrel.
Twin screws extruders may have cylindrical shaped bores in the extruder barrel to receive the screws. The screws of the extruder are most often arranged to intermesh with each other. The cylindrical bores of the barrel similarly overlap such that the bore has a figure-8 shape. Twin screw extruders may further have conical shaped screws and conical shaped bores complementing the shape of the screws. Conical extruders are suitable in systems requiring high pressures. The conical shape of the bore and the screws enable the higher pressures to be obtained more easily than in extruders with cylindrical bores and extruder screws.
The high pressures within the extruders and the resulting continuous wear have necessitated that wear resistant materials or coatings be applied to the internal surfaces of the extruder barrel. The surfaces are typically hardened by heat treatment, carburizing or nitriding and by applying a thin layer of wear-resistant material. The wear resistant coatings are usually applied by centrifugal casting at high temperatures or may be applied by welding. These processes are expensive and require significant finishing work after coating to obtain the necessary tolerances required for the extruder barrel. In the case of centrifugal casting, the resultant coating is subject to shrinkage and failure of the coating. Centrifugal casting and other wear surface treating processes are particularly difficult to apply to conical bores and bores with complex shapes.
Several processes have been developed to produce a wear resistant surface on the internal surface of the extruder barrel and to produce a system to reduce the wear of the parts. For example, U.S. Pat. No. 4,875,847 discloses a twin screw extruder having a conical nose section. The discharge end of the extruder barrel includes a central arcuate wall to define a pair of side-by-side, tubular, frusto-conical discharge outlets. The conical discharge outlets are reported to provide more efficient pumping action and to provide a bearing-type support for each screw to reduce contact of the screws against the walls of the extruder barrel. The extruder barrel is formed from half-head sections where the upper and lower half sections are bolted together.
Another example of a wear resistant surface is illustrated in U.S. Pat. No. 4,327,859 disclosing a method of forming the wear-resistant surface in an extruder barrel. A sheet of soft metal and a sheet of hard wear resistant metal are metallurgically bonded together to form a laminate and shaped into a tubular blank having the softer metal facing outward. The blank is inserted into the extruder barrel in a coaxial relationship with the barrel. An explosive charge is detonated inside the blank to expand the blank into contact with the walls of the barrel and bond the soft metal to the barrel.
U.S. Pat. No. 4,385,876 relates to an extruder having a replaceable liner. The extruder barrel is of a clam-shell design where the liner sections are in two halves and complementary in shape to the two sections of barrel. The split sections of the barrel are clamped together by bolts for assembly and disassembly.
A further example of a twin screw extruder is disclosed in U.S. Pat. No. 4,308,655. This device includes a pair of opposing casing shells having a cylindrical inner face. Longitudinal guide bars having outwardly facing concave sections are positioned between the casing shells. The casing shells and the guide bars are clamped together by bolts to form the extruder barrel.
The above-noted methods of producing extruder barrels and wear-resistant surfaces in the barrels are often not suitable for extruder barrels having complex shapes. Extruder barrels having conical bores are generally bored or machined from a single block of metal. The complex shape is not receptive of a wear resistant surface by many conventional methods, such as centrifugal casting. The complex shape of the bore necessitates extensive finishing work to produce the barrel thereby contributing to the expense of the manufacturing process.
The present invention overcomes the limitations of the previous methods of producing an extruder barrel by providing a less costly method of producing the extruder barrel. The method of the invention further enables the formation of hard wear resistant surfaces on complex shaped bores without the requirement of extensive finishing operations.